Cellulose triacetate has served as the film support for photographic roll films for many years. Its use continues because it has a good balance of physical properties and particularly because of two quite useful properties. First, after being wound on a core or reel as a camera roll film and acquiring the usual curl or "core set", triacetate film relaxes and becomes almost completely flat during aqueous development. Second, it is easy to cut and to perforate.
A problem, however, with cellulose triacetate is that films of high quality can be made from it only by coating or "casting" from a solvent mixture. It softens at such a high temperature that films made from it by melt extrusion have excessive thermal decomposition and color formation. Solvent casting uses expensive solvent mixtures and employs expensive solvent recovery equipment to comply with environmental requirements.
Melt-extruded poly(ethylene terephthalate) film has been used as a support for sheet films such as X-ray films. As a roll film support, however, it has two important drawbacks. First, it is too tough. Consequently, it is difficult to slit and perforate and it stretches rather than breaks when put under stress. Second, it does not quickly relax its core set or curl when processed in warm aqueous solutions. This means that the poly(ethylene terephthalate) film, which develops core set when wound around a film core, does not become flat after development and drying.
A need exists for a polymer from which roll films of photographic quality can be made without the use of solvents. The present invention provides such a polymer, from which films can be made by melt extrusion rather than solvent casting, which films meet the stringent requirements of photographic films, including being easy to cut and perforate and exhibiting curl relaxation after aqueous processing.